Single-element electron-transfer optical detector system

ABSTRACT

An optical detector system includes an electrically resistive screen that is substantially transparent to radiation energy having a wavelength of interest. An electron transfer element (e.g., a low work function photoactive material or a carbon nanotube (CNT)-based element) has a first end and a second end with its first end spaced apart from the screen by an evacuated gap. When radiation energy passes through the screen with a bias voltage being applied thereto, transfer of electrons through the electron transfer element is induced from its first to its second end such that a quantity indicative of the electrons transferred can be detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This patent application is co-pending with one related patentapplication entitled “MULTI-ELEMENT ELECTRON-TRANSFER OPTICAL DETECTORSYSTEM” (NASA Case No. LAR 16279-2), by the same inventor as this patentapplication.

ORIGIN OF THE INVENTION

[0002] The invention described herein was made by an employee of theUnited States Government and may be manufactured and used by or for theGovernment for governmental purposes without the payment of anyroyalties thereon or therefor.

BACKGROUND OF THE INVENTION

[0003] Field of the Invention

[0004] This invention relates to optical detectors. More specifically,the invention is a single-element optical detector system for imaging orsensing applications that detects and/or measures photo-induced electrontransfer through a micro-scale electron conduction element (e.g., acarbon nanotube (CNT), a photoactive material having a low workfunction, or a combination of these two) that are spaced from a biasvoltage screen through which radiation of interest passes.

SUMMARY OF THE INVENTION

[0005] In accordance with the present invention, an optical detectorsystem includes an electrically resistive screen that is substantiallytransparent to radiation energy having a wavelength of interest. Avoltage source is provided to apply a bias voltage to the screen. Anelectron transfer element (e.g., a low work function photoactivematerial, a carbon nanotube (CNT), or a CNT topped with a low workfunction photoactive material) has a first end and a second end with itsfirst end spaced apart from the screen by an evacuated gap. Whenradiation energy passes through the screen with the bias voltage beingapplied thereto, transfer of electrons through the electron transferelement is induced from its first to its second end. A detector,electrically coupled to the second end of the electron transfer element,detects a quantity indicative of the electrons transferred through theelectron transfer element. The optical detector system can operate asdescribed for imaging applications and can be adapted for sensingapplications by providing an analyte-sensitive, luminescent coating onthe screen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a schematic view of a single-element optical detectorsystem constructed in accordance with the present invention;

[0007]FIG. 2 is a schematic view of one embodiment of a single-elementoptical detector system having a carbon nanotube (CNT) electron transferelement;

[0008]FIG. 3 is a schematic view of another embodiment of asingle-element optical detector system having its electron transferelement constructed from a CNT topped with a low-work functionphotoactive material;

[0009]FIG. 4 is a schematic plan view of an embodiment of amulti-element imaging system constructed from single-element opticaldetector systems;

[0010]FIG. 5 is a schematic view of the multi-element imaging systemusing a common resistance screen and a focusing-lens;

[0011]FIG. 6 is a schematic plan view of a single-element opticaldetector system capable of sensing an analyte of interest;

[0012]FIG. 7 is a schematic plan view of an embodiment of amulti-element sensing system constructed from single-element opticaldetector systems;

[0013]FIG. 8 is a schematic plan view of a multi-element system thatcombines imaging and sensing capabilities; and

[0014]FIG. 9 is a schematic side view of the combined imaging andsensing multi-element system.

DETAILED DESCRIPTION OF THE INVENTION

[0015] Referring now to the drawings, and more particularly to FIG. 1, asingle-element optical detector system in accordance with an embodimentof the present invention is shown and referenced generally by numeral10. Optical detector system 10 forms the basic structure used toconstruct novel imaging elements/systems, sensing elements/systems, orcombined imaging and sensing systems, each of which will be describedfurther below.

[0016] Optical detector system 10 has a resistance screen 12 disposed inthe path of radiation energy 14 that is to be detected and, if desired,measured. Radiation energy 14 can be any visible or invisible lightenergy. Resistance screen 12 is any electrically resistive material(e.g., electrical resistivity of approximately 60 ohms per square orgreater) that is fully transparent or at least substantially transparentto the wavelength range of radiation energy 14. Electrically coupled toresistance screen 12 is a voltage source 16 that applies a biasingvoltage to resistance screen 12. Spaced apart from resistance screen 12is an electron transfer element 18 that can transfer electronstherethrough after it is exposed to radiation energy 14. As will beexplained further below, element 18 is preferably a carbon nanotube(CNT) based element, but could also be any photoactive material capableof photo-induced electron transfer. A small gap 20 (i.e., on the orderof 100 microns or less) is defined between resistance screen 12 and oneend 18A of electron transfer element 18. For reasons that will beexplained further below, gap 20 is preferably evacuated. The opposingend 18B of electron transfer element 18 is electrically coupled to ameasuring device 22 which can be an ammeter that measures current, avoltmeter that measures voltage, an electron counter that countselectrons reaching end 18B, or a device that measures any combination ofcurrent, voltage and electron counts.

[0017] In operation, a bias voltage is applied to resistance screen 12while it is exposed to incident radiation energy 14. Radiation energy 14passes through resistance screen 12 and is incident on electron transferelement 18 at end 18A thereof. The impingement of radiation energy 14 onelectron transfer element 18 induces photoelectron transporttherethrough, i.e., electron loss in element 18. The resulting electronvacancies or “holes” left in element 18 are filled with electronssourced from the voltage-biased resistance screen 12 resulting in acurrent flow. The photo-induced electron transfer through element 18 ismeasured at its end 18B. Evacuation of gap 20 minimizes electroncollisions in gap 20 thereby ensuring that the vast majority ofelectrons released from screen 12 will be transferred to element 18. Theelectron transfer is then detected/measured at end 18B by measuringdevice 22.

[0018] As noted above, optical detector system 10 forms the basicstructure for an optical imaging element and system made from an arrayof such elements. At the heart of each basic structure is electrontransfer element 18 which, in general, can be any photoactive materialthat exhibits electron release/flow following optical interrogation bylight of the desired wavelength. The energy required to effect electronrelease is given by the band-gap energy for a material, and is typicallyexpressed in units of energy, e.g., electron volts. This measurement canbe converted to wavelength to provide a measure of the longestwavelength (i.e., lowest energy) photon that will induce thephotoelectric effect. Thus, electron transfer element 18 can be used totailor the sensitivity of optical detector system 10 to specificwavelengths of interest. For example, a photoactive material that doesnot require a large amount of energy (or “work function”) to induce therelease of electrons therefrom can be used to construct an opticaldetector system that is sensitive to very short wavelengths. The workfunction is the amount of energy required for an electron to be releasedand depends on the number of electron shells an atom has. That is, thegreater the number of shells, the less the work function. This is due towhat is known as the “weak force” which is the force that the nucleus ofan atom has to retain its electrons. The weak force decreases as thedistance from the electron to the atom's nucleus increases. As anelectron's weak force decreases, so does the energy needed to free itfrom the atom.

[0019] Photoactive materials having low work functions include, forexample, cesiated metals such as cesiated silver oxide (AgOCs), cesiatedsodium potassium antimony ([Cs]Na2KSb), and cesiated antimony (SbCs).Other low work function materials include certain semi-conductormaterials such as indium gallium arsenic phosphide (InGaAsP), galliumarsenide (GaAs) and sodium potassium antimony (Na2KSb). Note that eachof the above-described materials has a preferable wavelength orbandwidth at which electron transfer therethrough is optimized.

[0020] To construct optical detector systems in accordance with thepresent invention that have a high-degree of spatial resolution,sensitivity and bandwidth, it is preferred that electron transferelement 18 be constructed partially or totally from a carbon nanotube(CNT). As is known in the art, CNTs are longitudinally-extending carbonfibril structures having a high electrical conductivity. A variety ofknown growth techniques can be used to construct single-wall (SW) andmulti-wall (MW) CNTs having diameters as small as one nanometer.Accordingly, FIG. 2 illustrates a single imaging element 30 using a CNT.Common reference numerals will be used for elements already describedabove.

[0021] In single imaging element 30, resistance screen 12 (biased by abiasing voltage from source 16) is exposed to radiation energy 14.Resistance screen 12 can be realized by a substantially transparent wiremesh screen. However, if absolute transparency over a broad wavelengthspectrum is desired, resistance screen 12 can be realized by a sheet ofindium tin oxide. Spaced apart from resistance screen 12 by gap 20 is aCNT 32 positioned such that its longitudinal axis 34 is substantiallyperpendicular to resistance screen 12. Note that system 30 will stillfunction if longitudinal axis 34 is not perpendicular to resistancescreen 12, although some electron transfer losses may occur. CNT 32 isrepresentative of a single-wall CNT (SWCNT) or a multi-wall (MWCNT).Although current fabrication techniques favor MWCNTs over SWCNTs (owingto the complexities associated with controlling alignment of SWCNTsduring the growth thereof), it is to be understood that the presentinvention can use either type.

[0022] The gap 20 between resistance screen 12 and one longitudinal end32A of CNT 32 is on the order of 100 microns or less. To provide forevacuation of gap 20, an evacuated chamber 36 can be provided to encloseresistance screen 12 and CNT 32 with screen 12 still being capable ofhaving radiation energy 14 impinge thereon. A metal electrode 38 can becoupled to the other longitudinal end 32B of CNT 32 to provide ameasurement point for measuring device 22.

[0023] If it is desired to be sensitive to a particular wavelength bandof radiation energy 14, the optical detector system can be constructedas illustrated in FIG. 3. More specifically, optical detector system 40is identical to system 30 except that end 32A of CNT 32 is “topped” witha layer of a low work function photoactive material (PA) 42, several ofwhich were described above. In this way, electron transfer through CNT32 is induced only by the presence of a radiation energy 14 that is inthe wavelength region to which photoactive material 42 is sensitive.

[0024] Each of the above-described optical detector system “elements”can be used in an imaging system constructed from an array of suchelements. This embodiment is illustrated schematically in FIG. 4 wherean array 50 of optical detector elements based on system 10 areprovided. In this plan view, the resistance sheet is omitted for clarityof illustration so that each of electron transfer elements 18 isvisible. Note that systems 30 or 40 could also be used to constructarray 50. Further, a combination of systems 10, 30 and 40 could be usedto construct such an imaging array. In this way, portions of the imagingarray could be made more sensitive to a particular wavelength region ofincoming radiation energy. Also, note that an image pixel can be formedby one or more of electron transfer elements 18.

[0025] Each element 18 is uniquely addressable and can have its electrontransfer amounts detected/measured by addressing measurement device 52.That is, measurement device 52 functions as an individual measuringdevice (analogous to measurement device 22 described above) for all ofelements 18. The simultaneously-read outputs from device 52 can beprovided to a display 54.

[0026] Although each element in an imaging array can be constructedindividually, some economy of scale can be applied in the constructionprocess without departing from the scope of the present invention. Forexample, as illustrated in FIG. 5, a single resistance screen 12 can beused to span gap 20 between ends 32A of an array of CNTs 32 that residein a common evacuated chamber 36. Further, if necessary, a focusing lens56 can be placed in front of resistance screen 12 to bring an imagedarea/object into focus.

[0027] The present invention can also be used in a sensing capacity. Toillustrate this, optical detector system 10 (FIG. 1) has been modifiedto yield optical detector system 60 shown in FIG. 6 that can sense ananalyte of interest. However, it is to be understood that either ofoptical detector systems 30 and 40 could be similarly modified withoutdeparting from the scope of the present invention. As used herein,“analyte” means any gas or liquid-phase species for which an opticaltransduction mechanism exists or could be developed.

[0028] Optical detector system 60 includes a luminescent coating layer62 deposited on resistance screen 12. Layer 62 is representative of anoptical transduction mechanism and is generally realized by any coatingthat experiences changes in luminescence (e.g., brightness, color,excited state lifetime) in the presence of a particular analyte 15.Because some luminescent coatings must be optically excited duringoperation thereof, a light source 64 can be coupled to layer 62. Ofcourse, the excitation light source could be integrated or incorporatedin layer 62. Still further, in an array of such detectors, a singlelight source could be used to excite the luminescent coating(s).

[0029] Operation of optical detector system 60 is similar to that ofoptical detector system 10, except that changes in luminescence ofcoating layer 62 would also be quantified. Specifically, the change inluminescence brought about by the concentration of analyte 15 will causeelectron transfer through element 18 to increase or decrease. Suchchanges are recorded at measuring device 22.

[0030] A sensing array based on a plurality of optical detector systems60 can be constructed in a fashion similar to the construction of theimaging array. By way of example, this embodiment is illustrated in FIG.7 where an array 70 of optical detector elements based on system 60 areprovided. Although not illustrated in FIG. 7, a common resistance screenand gap can be provided above each electron transfer element 18 similarto the construction shown in FIG. 5. Further, a single commonluminescent coating layer could be provided above the entire array 70.However, array 70 can also be constructed to sense/measure changes in avariety of analyte concentrations. That is, in the illustratedembodiment example, array 70 is formed with a plurality of differentluminescent coating materials 62A-62E. Note that elements 18 are shownin phantom to illustrate their position under coating materials 62A-62E.Each of the luminescent coating materials 62A-62E can be associated withone or more elements 18. For example, as is shown in FIG. 7 as coatingmaterial 62E, the more sensitive coating materials may require only oneelement 18 to achieve the particular analyte sensing/measurement.

[0031] The present invention can be further adapted to provide acombination imaging and sensing array optical detector system. One suchcombination system is illustrated by way of example in FIGS. 8 and 9,where like reference numerals are used for elements already describedherein. In the plan view shown in FIG. 8, an array 80 is constructedwith an inner arrangement 80A of imaging elements based on, for example,system 30, and a surrounding arrangement 80B of sensing elements basedon system 60. Once again, for clarity of illustration, the plan view ofFIG. 8 omits the illustration of the resistance screen and subsequentevacuated gap between the electron transfer elements. Note that thesurrounding luminescent coating layer can comprise different coatingmaterials 62F-62I sensitive to different analytes. The above-describedcombined imaging and sensing construction is also illustrated in a sideview in FIG. 9. The same resistance screen 12 can be used across theentirety of array 80 (i.e., over all elements 18 and CNTs 32) whilefocusing lens 56 is positioned over inner arrangement 80A of imagingelements based on CNTs 32.

[0032] The device architecture of the present invention exploits theunique properties of CNTs (e.g., high strength-to-mass ratio and highelectrical conductivity) in the development of an imaging and sensingplatform with abundant spatial resolution and extremely high bandwidth(e.g., in excess of one gigahertz). The present invention may be wellsuited for large-scale production due to its simple operational concept.As the level of sophistication and control over the growth and alignmentof single-wall CNTs (SWCNT) increases, additional advances in theCNT-based imaging and sensing device will be realized due to the greaterstrength and conductivity of SWCNTs versus their MWCNT counterparts.

[0033] The present invention can be used for scientific, industrial andrecreational imaging science. Further, the development of CNT-basedimaging technology coupled with near-field microscopy could be used forthe biological and immunological sciences. Finally, the concept ofnanoscopic imaging and sensing elements lends itself to the productionof space-suitable systems based on their low weight, high (information)density, low power consumption, and high bandwidth.

[0034] Although only a few exemplary embodiments of this invention havebeen described in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. In the claims, means-plus-function andstep-plus-function clauses are intended to cover the structures or actsdescribed herein as performing the recited function and not onlystructural equivalents, but also equivalent structures. Thus, although anail and a screw may not be structural equivalents in that a nailemploys a cylindrical surface to secure wooden parts together, whereas ascrew employs a helical surface, in the environment of fastening woodenparts, a nail and a screw may be equivalent structures.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. An optical detector system, comprising: meansfor supplying electrons, said means for supplying electrons beingsubstantially transparent to radiation energy having a wavelength ofinterest, said means for supplying also being electrically resistive; avoltage source for applying a bias voltage to said means for supplying;means for transferring electrons, said means for transferring having afirst end and a second end, said means for transferring having saidfirst end thereof spaced apart from said means for supplying electronsby an evacuated gap wherein, when the radiation energy passes throughsaid means for supplying with the bias voltage being applied thereto,transfer of electrons through said means for transferring is inducedfrom said first end to said second end thereof; and detection means,electrically coupled to said second end of said means for transferring,for detecting a quantity indicative of the electrons transferred throughsaid means for transferring.
 2. An optical detector system as in claim 1further comprising a luminescent-based, analyte-sensing means positionedto have the radiation energy pass therethrough before impingement onsaid means for supplying, said analyte-sensing means changing in termsof at least one luminescent property in the presence of an analyte ofinterest.
 3. An optical detector system as in claim 1 wherein said meansfor supplying electrons is a material having an electrical resistivityof at least approximately 60 ohms per square.
 4. An optical detectorsystem as in claim 1 wherein said means for supplying electrons isindium tin oxide.
 5. An optical detector system as in claim 1 whereinsaid means for transferring electrons comprises a photoactive materialthat experiences electron release in the presence of radiation energy.6. An optical detector system as in claim 1 wherein said means fortransferring electrons comprises a carbon nanotube (CNT) having alongitudinal axis extending between said first end and said second endof said means for transferring electrons.
 7. An optical detector systemas in claim 6 wherein said CNT is a multi-wall carbon nanotube (MWCNT).8. An optical detector system as in claim 1 wherein said means fortransferring electrons comprises: a carbon nanotube (CNT) having alongitudinal axis extending between a first end and a second end of saidCNT, wherein said second end of said CNT forms said second end of saidmeans for transferring electrons; and a photoactive material positionedon said first end of said CNT wherein said photoactive material formssaid first end of said means for transferring electrons.
 9. An opticaldetector system as in claim 8 wherein said CNT is a multi-wall carbonnanotube (MWCNT).
 10. An optical detector system as in claim 1 whereinsaid detection means comprises an ammeter.
 11. An optical detectorsystem as in claim 1 wherein said detection means comprises a voltmeter.12. An optical detector system as in claim 1 wherein said detectionmeans comprises means for counting the number of electrons reaching saidsecond end of said second means.
 13. An optical detector system as inclaim 1 further comprising an evacuated chamber enclosing said means forsupplying electrons and said means for transferring, said evacuatedchamber having a portion thereof transparent to the radiation energy andpositioned to permit impingement of the radiation energy on said meansfor supplying.
 14. An optical detector system, comprising: resistancemeans transparent to radiation energy having a wavelength of interestand being electrically resistive; a voltage source for applying a biasvoltage to said resistance means; a carbon nanotube (CNT)-based elementhaving a longitudinal axis extending between a first end and a secondend thereof, said CNT-based element being capable of electron transfertherethrough, said CNT-based element positioned with the longitudinalaxis approximately perpendicular to said resistance means with saidfirst end thereof spaced apart from said resistance means by anevacuated gap wherein, when the radiation energy passes through saidresistance means with the bias voltage being applied thereto, transferof electrons through said CNT-based element is induced from said firstend to said second end thereof; and detection means, electricallycoupled to said second end of said CNT-based element, for detecting aquantity indicative of the electrons transferred through said CNT-basedelement.
 15. An optical detector system as in claim 14 furthercomprising a luminescent-based, analyte-sensing means positioned to havethe radiation energy pass therethrough before impingement on saidresistance means, said analyte-sensing means changing in terms of atleast one luminescent property in the presence of an analyte ofinterest.
 16. An optical detector system as in claim 14 wherein saidresistance means is a material having an electrical resistivity of atleast approximately 60 ohms per square.
 17. An optical detector systemas in claim 14 wherein said resistance means is indium tin oxide.
 18. Anoptical detector system as in claim 14 wherein said CNT-based elementcomprises a multi-wall carbon nanotube (MWCNT).
 19. An optical detectorsystem as in claim 14 wherein said CNT-based element comprises: a CNT;and a photoactive material positioned on said CNT to form said first endof said CNT-based element.
 20. An optical detector system as in claim 19wherein said CNT is a multi-wall carbon nanotube (MWCNT).
 21. An opticaldetector system as in claim 14 wherein said detection means comprises anammeter.
 22. An optical detector system as in claim 14 wherein saiddetection means comprises a voltmeter.
 23. An optical detector system asin claim 14 wherein said detection means comprises means for countingthe number of electrons reaching said second end of said second means.24. An optical detector system as in claim 14 further comprising anevacuated chamber enclosing said resistance means and said CNT-basedelement, said evacuated chamber having a portion thereof transparent toradiation energy and positioned to permit impingement of radiationenergy on said resistance means.
 25. An optical detector system,comprising: resistance means transparent to radiation energy having awavelength of interest and being electrically resistive; a voltagesource for applying a bias voltage to said resistance means; amulti-wall carbon nanotube (MWCNT) having a longitudinal axis extendingbetween a first end and a second end thereof, said MWCNT positioned withthe longitudinal axis approximately perpendicular to said resistancemeans with said first end thereof spaced apart from said resistancemeans by a gap; evacuation means cooperating with said gap for placingsaid gap in an evacuated state wherein, when radiation energy passesthrough said resistance means with the bias voltage being appliedthereto, transfer of electrons through said MWCNT is induced from saidfirst end to said second end thereof; and detection means, electricallycoupled to said second end of said MWCNT, for detecting a quantityindicative of the electrons transferred through said MWCNT.
 26. Anoptical detector system as in claim 25 further comprising aluminescent-based, analyte-sensing means positioned to have radiationenergy pass therethrough before impingement on said resistance means,said analyte-sensing means changing in terms of at least one luminescentproperty in the presence of an analyte of interest.
 27. An opticaldetector system as in claim 25 wherein said resistance means is amaterial having an electrical resistivity of at least approximately 60ohms per square.
 28. An optical detector system as in claim 25 whereinsaid resistance means is indium tin oxide.
 29. An optical detectorsystem as in claim 25 further comprising a photoactive materialpositioned on said first end of said multi-wall carbon nanotube.
 30. Anoptical detector system as in claim 25 wherein said detection meanscomprises an ammeter.
 31. An optical detector system as in claim 25wherein said detection means comprises a voltmeter.
 32. An opticaldetector system as in claim 25 wherein said detection means comprisesmeans for counting the number of electrons reaching said second end ofsaid multi-wall carbon nanotube.
 33. An optical detector system,comprising: resistance means transparent to radiation energy having awavelength of interest and being electrically resistive; a voltagesource for applying a bias voltage to said resistance means; asingle-wall carbon nanotube (SWCNT) having a longitudinal axis extendingbetween a first end and a second end thereof, said SWCNT positioned withthe longitudinal axis approximately perpendicular to said resistancemeans with said first end thereof spaced apart from said resistancemeans by a gap; evacuation means cooperating with said gap for placingsaid gap in an evacuated state wherein, when radiation energy passesthrough said resistance means with the bias voltage being appliedthereto, transfer of electrons through said SWCNT is induced from saidfirst end to said second end thereof; and detection means, electricallycoupled to said second end of said SWCNT, for detecting a quantityindicative of the electrons transferred through said SWCNT.
 34. Anoptical detector system as in claim 33 further comprising aluminescent-based, analyte-sensing means positioned to have radiationenergy pass therethrough before impingement on said resistance means,said analyte-sensing means changing in terms of at least one luminescentproperty in the presence of an analyte of interest.
 35. An opticaldetector system as in claim 33 wherein said resistance means is amaterial having an electrical resistivity of at least approximately 60ohms per square.
 36. An optical detector system as in claim 33 whereinsaid resistance means is indium tin oxide.
 37. An optical detectorsystem as in claim 33 further comprising a photoactive materialpositioned on said first end of said single-wall carbon nanotube.
 38. Anoptical detector system as in claim 33 wherein said detection meanscomprises an ammeter.
 39. An optical detector system as in claim 33wherein said detection means comprises a voltmeter.
 40. An opticaldetector system as in claim 33 wherein said detection means comprisesmeans for counting the number of electrons reaching said second end ofsaid single-wall carbon nanotube.